1. Field of the Invention
The present invention relates to vehicle speedometers, and more specifically to a vehicle speedometer wherein a measuring wave is transmitted towards a running road surface and then reflected by the road surface, and the vehicle speed is detected according to the Doppler effect of the reflected wave.
2. Description of the Prior Art
In known Doppler effect speedometers, microwaves are transmitted and received by an antenna installed at a predetermined angle with respect to the road surface. The microwave measuring wave is transmitted from the antenna and reflected at the road surface and the reflected wave is received. Variations in the frequency of the reflected wave relate to the vehicle speed according to the Doppler effect. Such a speedometer is advantageous in that error due to tire slip does not occur in comparison to prior art speedometers wherein vehicle speed is determined by monitoring the rotational speed of the vehicle drive shaft.
In such a Doppler type speedometer, however, the transmitting and receiving antenna moves vertically due to the vehicle vibration or due to variations in the angle of transmitting and receiving antenna with respect to the road surface caused by, for example, pitching vibration or nose-dive vibration. This introduces error in the measuring.
In order to eliminate the error due to the former cause, for example, Japanese examined patent publication No. 26912/1984 is disclosed. In this patent, a microwave transmitting and receiving antenna is installed in a definite angle with respect to the road surface at the front and rear of the vehicle. The received signals of reflected waves at the road surface, obtained by the transmitting and receiving antenna, are inputted to a phase shifter and then synthesized. Thus error caused by the vertical movement of the transmitting and receiving antenna is cancelled. However, no remedy has yet been proposed in order to eliminate error due to the latter cause.